Global Navigation Satellite Systems (GNSS) provide aircraft with navigation support in approach and landing operations. However, since the accuracy and precision requirements are high in approach and landing operations, Ground Based Augmentation Systems (GBAS) augment GNSS when an aircraft is near a GBAS Ground Subsystem. GBAS Ground Subsystems, also referred to herein as GBAS stations, augment GNSS receivers by broadcasting pseudorange corrections and integrity information to the aircraft, which helps remove GNSS errors impacting satellite measurements processed by the aircraft's GNSS receiver. As a result, aircraft can have improved continuity, availability, and integrity performance for precision approaches, departure procedures, and terminal area operations.
Ground Based Augmentation Systems (GBAS) are susceptible to spatial decorrelation errors between the ground subsystem (providing GPS corrections) and airborne subsystems (consuming GPS corrections) due to GPS satellite ephemeris faults. This ephemeris fault threat is mitigated via a ground broadcast ephemeris decorrelation parameter, which is applied in the airborne subsystem's integrity limit computations. The ground subsystem continually monitors (in real-time) this broadcast decorrelation parameter to ensure the broadcast value provides valid integrity bounding for the aircraft's navigation position solution.
Currently available GBAS stations include a first order hold (FOH) and maneuver detector monitors (MDM), which utilize stored ephemeris data as part of their real-time monitoring capability. The FOH and/or the MDM are tripped (i.e., triggered) when ephemeris data for a particular satellite is bad (faulty). When this occurs, the data saved for the faulty satellite, which has been accumulating for prior days, is eliminated and the faulty satellite is not used for the next two days. Taking one satellite off-line for two days could potentially cause loss of function of the GBAS station. If there are not enough satellites to give a good solution, the bounding error becomes large. Large bounding errors impact continuity and available requirements.
When a satellite is reintroduced to a satellite system (i.e., the satellite's status transitions from invalid to valid), the GBAS station is required to wait at least twenty-four hours for the ephemeris buffer to be filled.
When a GBAS station is brought on-line after being off-line for more than twenty-four hours, the GBAS station must wait for the ephemeris buffers for each satellite in view of the GBAS station to be filled with at least twenty-fours of data before that satellite can be used by the GBAS station. Likewise, if a GBAS station is out of commission for more than one day, the GBAS station must wait for the ephemeris buffers for each satellite in view of the GBAS station to be filled with at least twenty-fours of data before that GBAS station can be used.